The use of inkjet printing systems has grown dramatically in recent years, which may be attributed to substantial improvements in print resolution and overall print quality coupled with appreciable reduction in cost. Notwithstanding their recent success, intensive research and development efforts continue toward improving inkjet print quality. In general, inkjet print quality still falls short of that produced by more expensive technologies, such as photography and offset or gravure printing. A surge in interest in inkjet imaging (e.g., the rendition of pictures) has resulted in the need to produce near photographic quality printed images at a reasonable cost. The challenge remains to further improve the image quality and lightfastness of inkjet printed images, especially images containing skin tones, without increasing their cost.
Color inkjet printers typically use four inkjet inks: cyan, yellow, magenta, and black. To provide a desired color or hue, each of the inkjet inks includes a colorant, such as a pigment and/or dye, dispersed or dissolved in an ink vehicle. A perceived color is typically described using a color space, such as Commission Internationale de l'Eclairage (“CIELAB”) or Munsell, which are well known in the art. For example, in the Munsell color space, a particular color is defined using three terms: hue, value, and chroma. In the CIELAB color space, a particular color is defined by three terms: L*, a*, and b*. L* defines the lightness of a particular color and typical L* values range from zero (black) to 100 (white). The terms a* and b*, together, define the hue and chroma of a particular color. Chroma, C*, is defined according to the equation C*=[(a*)2+(b*)2)])1/2. The term a* ranges from a negative number (green) to a positive number (red). The term b* ranges from a negative number (blue) to a positive number (yellow).
As used herein, the term “lightfastness” refers to the ability of the inkjet ink to retain its chromaticity and/or optical density after exposure to light. Lightfastness is typically measured by an amount of time to failure, such as years to failure. The failure occurs after a specific percentage of loss in optical density. Typically, highly chromatic pigments exhibit poor lightfastness. In other words, there is generally a tradeoff between the chroma of the pigment and its lightfastness.
Another attribute that is important in producing photographic quality printed images is grain. As used herein, the term “grain” refers to how visible the dots of the inkjet ink are to a viewer. If large, dark drops of the inkjet ink are used, the dots are very visible and image quality is degraded. One solution for reducing grain is to apply smaller drop weights to produce nearly invisible drops. However, this technique results in a more difficult and more costly manufacturing process because smaller orifice holes and resistors are utilized. In addition, reliability is decreased due to the smaller holes.
Another solution to reduce grain is to apply a “light” inkjet ink, such as a light magenta or light cyan, in combination with the normal, or dark, inkjet inks. The light inkjet ink typically has a colorant loading that ranges from 5% to 50% of the colorant loading of the dark inkjet ink. This technique allows larger, low colorant load drops of the inkjet ink to be used. The drops of the light inkjet ink are less visible to the viewer due to the presence of less colorant. However, one disadvantage of this solution is that the light inkjet ink fades much faster on the print medium than the dark inkjet ink. This fade often occurs in critical areas, such as in the skin tone of a human being. To overcome the fading problem, a protective overlaminate is applied to the printed image. However, applying an overlaminate is not an economically feasible option with low cost printers and, therefore, the performance of the output of these printers is limited by graininess, differential lightfade, or a combination thereof. Alternatively, to reduce the lightfade, the graininess or pixilation of the printed image is increased.
As the demand for photographic quality images continues to increase, there is a need for developing inkjet inks that provide printed images that have high chromaticity and high lightfastness with reduced graininess.